U.S. patent number 4,092,060 [Application Number 05/665,434] was granted by the patent office on 1978-05-30 for thin film optical switching device.
This patent grant is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Yoshinori Nomura, Masahiro Nunoshita.
United States Patent |
4,092,060 |
Nunoshita , et al. |
May 30, 1978 |
Thin film optical switching device
Abstract
A thin-film optical switching device has a substrate of fused
quartz, an optical slab waveguide or Corning glass No. 7059, a thin
film crossing a portion of the length of the waveguide, and an
interdigital transducer on the substrate for generating an elastic
surface wave to propagate it through the thin film. The optical
waveguide, and thin film have respective parameters such that
almost all energy of an optically guided wave and the elastic
surface wave are concentrated into the thin film while the
optically guided wave within the thin film undergoes
acousto-optical interaction, that is, Bragg diffraction.
Inventors: |
Nunoshita; Masahiro (Amagasaki,
JA), Nomura; Yoshinori (Amagasaki, JA) |
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha (Tokyo, JA)
|
Family
ID: |
12588212 |
Appl.
No.: |
05/665,434 |
Filed: |
March 10, 1976 |
Foreign Application Priority Data
|
|
|
|
|
Apr 2, 1975 [JA] |
|
|
50-40713 |
|
Current U.S.
Class: |
385/7;
385/16 |
Current CPC
Class: |
G02F
1/335 (20130101); G02F 1/3137 (20130101) |
Current International
Class: |
G02F
1/29 (20060101); G02F 1/335 (20060101); G02F
1/313 (20060101); G02B 005/14 () |
Field of
Search: |
;350/96WG,96C |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Kuhn et al., article in Applied Physics Letter, Sep. 15, 1970, pp.
265-267. .
Shah article in Applied Physics Letter, Jul. 15, 1973, pp. 75-77.
.
Ohmachi article in Journal of Applied Physics, Sep. 1973, pp.
3928-3933. .
Suematsu et al., article in IEEE Journal of Quantum Electronics,
QE-10, Feb. 1974, pp. 222-229. .
Leon et al., article in IBM Tech. Disc. Bull., Jan. 1973, p.
2630..
|
Primary Examiner: Corbin; John K.
Assistant Examiner: Hille; Rolf
Claims
What we claim is:
1. A thin-film optical switching device comprising, in combination,
a substrate having a first refractive index, an optical waveguide
disposed on said substrate, said optical waveguide having a width
sufficiently larger than that of an optical guided wave and also
sufficient flare to prevent the optical guided wave from disturbing
a diffracted guided optical wave, and further having a second
refractive index higher than said first refractive index and a
first film thickness, a thin film member overlying at least one
portion of said optical waveguide, said thin film member extending
in a light propagation direction and a distance from the point
where the waveguide starts to flare greater than the width of the
sound wave and having a third refractive index higher than said
second refractive index, a relatively large value of
acousto-optical figure-of-merit and a second film thickness, and an
electromechanical transducer means on said substrate and positioned
laterally of said waveguide for generating an elastic surface wave
to propagate it through said thin film member, said transducer
being oriented so that an angle .theta. formed between the wave
front of an elastic surface wave generated by said transducer and
an incident guided optical wave fulfills the requirement for the
Bragg diffraction
where n designates the effective refractive index for the optical
guided wave, .LAMBDA. the wavelength of the elastic surface wave
and .lambda.o designates the wavelength of light in a vacuum, said
second refractive index and first film thickness of said optical
waveguide and said third refractive index and second film thickness
of said thin-film member having respective values for imparting to
the device a sufficiently high diffraction efficiency at an optical
wavelength used with the device.
2. A thin-film optical switching device as claimed in claim 1
wherein said optical waveguide is a glass having the composition
61.0 mol % of SiO.sub.2, 10.0 mol % of Al.sub.2 O.sub.3, 19.0 mol %
of B.sub.2 O.sub.3 and 10.0 mol % of BaO, said glass being
sputtered onto said first film thickness on said substrate.
3. A thin-film optical switching device as claimed in claim 1
wherein said substrate is a piezoelectric ferroelectric
crystal.
4. A thin-film optical switching device as claimed in claim 1
wherein said substrate is a nonpiezoelectric material having a
relatively small acousto-optical effect.
5. A thin-film optical switching device as claimed in claim 1
wherein said thin-film member is a glass selected from the group
consisting of arsenic sulfide As.sub.2 Sx where x has a value of
from 3 to 7, and tellurium dioxide (TeO.sub.2) glass.
6. A thin-film optical switching device as claimed in claim 3
wherein said piezoelectric, ferroelectric crystal is a compound
selected from the group consisting of lithium niobate (LiNbO.sub.3)
and lithium tantalate (LiTaO.sub.3), and said optical waveguide is
formed by diffusing an element selected from the group consisting
of lithium (Li) niobium (Nb), titanium (Ti) and copper (Cu) on said
substrate of said piezoelectric ferroelectric crystal.
7. A thin-film optical switching device as claimed in claim 1
wherein said substrate is a ferroelectric crystal and said optical
waveguide is a single crystal in the form of a thin-film formed on
said crystal substrate by an epitaxial growth technique.
8. A thin-film optical switching device as claimed in claim 1
wherein said thin film member has at each of the opposite edges in
the direction in which an optically guided wave travels within said
optical waveguide a slope not more than 1/1000.
9. A thin-film optical switching device comprising, in combination,
a substrate of fused quartz having a first refractive index
n.sub.o, an optical waveguide disposed on said substrate having a
width sufficiently larger than that of an optical guided wave and
also sufficient flare to prevent the optical guided wave from
disturbing a diffracted guided optical wave and being a glass
having the composition 61.0 mol % of SiO.sub.2, 10.0 mol % of
Al.sub.2 O.sub.3, 19.0 mol % of B.sub.2 O.sub.3 and 10.0 mol % of
BaO, said glass having a second refractive index n, a thin-film
member overlying at least one portion of said optical waveguide
extending in a light propagation direction and a distance from the
point where the waveguide starts to flare greater than the width of
the sound wave and being of a chalcogenide glass of the As.sub.2 Sx
type, where x has a value of from 3 to 7, and having a third
refractive index n.sub.2, said refractive indices being in the
relationship n.sub.0 <n.sub.1 <n.sub.2, and an
electromechanical transducer means on said substrate and positioned
laterally of said waveguide for generating an elastic surface wave
for propagating through said thin-film member, said transducer
being oriented so that an angle .theta. formed between the wave
front of an elastic surface wave generated by said transducer and
an incident guided optical wave fulfills the requirement for the
Bragg diffraction
where n designates the effective refractive index for the optical
guided wave, .LAMBDA. the wavelength of the elastic surface wave
and .lambda.o designates the wavelength of light in a vacuum.
10. A thin-film optical switching device comprising, in
combination, a substrate of fused quartz having a refractive index
of 1.45 at an optical wavelength of 8,700 A, an optical waveguide
disposed on said substrate and having a width sufficiently larger
than that of an optical guided wave and also sufficient flare to
prevent the optical guided wave from disturbing a diffracted guided
optical wave, and a film thickness of from 1.0 to 1.5 .mu.m, said
optical waveguide being a glass having the composition 61.0 mol %
of SiO.sub.2, 10.0 mol % of Al.sub.2 O.sub.3, 19.0 mol % of B.sub.2
O.sub.3 and 10.0 mol % of BaO, said glass having a refractive index
of 1.53 at said wavelength, a thin-film member overlying at least
one portion of said optical waveguide and extending in a light
propagation direction and a distance from the point where the
waveguide starts to flare greater than the width of the sound wave
and having a thickness of from 2,000 to 3,000 A, said thin-film
member being of a chalcogenide glass of the As.sub.2 Sx type where
x has a value of from 3 to 7 having a refractive index of 2.35 at
said wavelength, said thin-film member having at each of the
opposite edges thereof in a direction in which an optically guided
wave travels within said optical waveguide a slope not higher than
1/1000, and an electromechanical transducer means on said substrate
and positioned laterally of said waveguide for generating an
elastic surface wave for propagating through said thin-film member,
said transducer being oriented so that an angle .theta. formed
between the wave front of an elastic surface wave generated by said
transducer and an incident guided optical wave fulfills the
requirement for the Bragg diffraction
where n designates the effective refractive index for the optical
guided wave, .LAMBDA. the wavelength of the elastic surface wave
and .lambda.o designates the wavelength of light in a vacuum.
Description
BACKGROUND OF THE INVENTION
This invention relates to improvements in a thin-film optical
switching device.
In order to put systems for processing and communicating optical
information by the use of laser light to practical use, the optical
circuit elements should be combined into an integrated circuit. If
mechanical means for rotating a mirror or a prism is unavailable,
it is possible to modulate or deflect an optical wave by changing
the refractive index of the particular medium through which the
optical wave travels. This measure can be achieved by utilizing the
electro-optical effect, magneto-optical effect or acousto-optical
effect according to which the substance involved changes in
refractive index. For example, there have been already proposed
thin-film optical switching and modulating devices utilizing such
effects.
Among them thin-film optical switching devices utilizing the
acousto-optical effect are promising because they can employ a
substrate of noncrystalline material, for example, glass, as long
as the substrate is operatively associated with a suitable
transducer for generating an elastic surface wave and because there
is no necessity for using a thin film composed of a special
electro-optical or magneto-optical crystal for the wave guide and
the substrate. Among optical switching devices utilizing an elestic
surface wave there are well known (a) optical switching devices
including a quartz substrate and a thin-film optical waveguide
formed by sputtering Corning glass No. 7059 upon the substrate, (b)
those including a substrate of ferroelectric lithium niobate
(LiNbO.sub.3) and a thin-film optical waveguide formed by
sputtering chalcognide glass on the substrate or by diffusing a
suitable metal or its oxide into the surface layer of the
substrate, (c) those including a substrate made of fused quartz or
glass available under the trade mark PYREX and an
electro-mechanical transducer with interdigital electrodes and an
optical waveguide each formed of a thin film of zinc oxide (ZnO)
disposed on the substrate.
The devices (a) have had low light deflection or diffraction
efficiency, and the devices (b) have had a high optical propagation
loss while they have not been suitable for efficiently coupling to
other optical circuit elements not requiring a substrate of
ferroelectric crystals. Also the devices (c) have had a high
optical propagation loss and a low light diffraction efficiency.
Thus all the devices as above described have been disadvantageous
in that it is difficult to simultaneously have low propagation and
coupling loss and high diffraction efficiencies. In other words,
such devices have not been practically available far integrated
optical circuits.
Accordingly it is a general object of the present invention to
eliminate the disadvantages of optical switching elements of the
conventional types as above described.
It is an object of the present invention to provide a new and
improved thin-film optical switching device having a low light
propagation loss and high light diffraction efficiency yet which is
still easy to couple to and integrate with other thin-film optical
circuit elements.
SUMMARY OF THE INVENTION
The present invention provides a thin-film optical switching device
comprising a substrate, an optical slab waveguide disposed on the
substrate, and a thin film overlapping on at least one portion of
the optical slab waveguide. The thin film is formed of a
transparent material having a higher refractive index and greater
acousto-optical figure-of-merit than the material forming the
optical waveguide. The optical waveguide and thin film have
respective refractive indices and film thicknesses determined in
accordance with the wavelength of light used with the device so
that that portion of the optical slab waveguide overlaid by the
thin film has a propagation constant which is as high as possible.
Further an electro-mechanical transducer is disposed on the
substrate to generate an elastic surface wave for propagating along
the surface of the thin film. In the device thus formed, an optical
guided wave propagating through the optical waveguide is introduced
into the thin film through that portion of the optical waveguide
overlaid by the thin film. Within the thin film the elastic surface
wave from the transducer acts on the introduced optical wave to
deflect the path of the optical guided wave whereby the switching
of the optical guided wave is accomplished with both a low
propagation loss and a high light diffraction efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more readily apparent from the
following detailed description taken in conjunction with the
accompanying drawings in which:
FIG. 1 is a fragmental perspective view of a thin-film optical
switching device constructed in accordance with the principles of
the prior art with an associated electric circuit illustrated in
the form of a block diagram.
FIG. 2 is a fragmental cross sectional view taken along the line
II--II of FIG. 1;
FIG. 3 is a fragmental perspective view of a thin-film optical
switching device constructed in accordance with the principles of
the present invention with an associated electric circuit
illustrated in the form of a block diagram;
FIG. 4 is a fragmental cross sectional view taken along the line
IV--IV of FIG. 3; and
FIG. 5 is another fragmental cross sectional view taken along the
line V--V of FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings in detail, FIGS. 1 and 2 show a
thin-film optical switching device constructed in accordance with
the principles of the prior art and belonging to the category of
the devices (c) as above described. The arrangement illustrated
comprises a substrate 10 formed, for example, of fused quartz, an
optical slab waveguide 12 disposed on one surface of the substrate
10 along the longitudinal axis thereof and extending throughout the
length thereof. The optical waveguide 12 includes one half, in this
case, a lefthand half as viewed in FIG. 1 having a uniform width
and the other or righthand half flared toward its free end. The
optical waveguide 12 is a thin-film formed by sputtering Corning
glass No. 7059 onto the substrate 10 by a photolithographic
technique. The composition of Corning glass No. 7059 is as
disclosed by T. Nishimura et al. in an article entitled "Thin Glass
Film Optical Waveguides", Reports of Researches read in the
subcommittee meeting of Electronic Quantum-Statistical Theory of
Matter held on July 25, 1973 in Tokyo. The arrangement further
includes a rectangular thin film 14 of any suitable piezoelectric
material such as zinc oxide (Zn), zinc sulfide (ZnS) or the like
disposed on substrate 10 adjacent to one longer edge. The
piezoelectric thin film 14 serves as an electro-mechanical
transducer for generating an elastic surface wave.
As shown in FIG. 1, a pair of interdigital electrodes 16 are
disposed on the one surface of the substrate 10 and connected to
the transducer 14 and electrically connected across a source of
high frequency voltage 18 through a matching circuit 20.
In operation a high frequency voltage from the source 18 is applied
across the electrodes 16 and therefore to the interdigital
transducer 14 through the matching circuit 20 to generate an
elastic surface wave 22 and propagate it along the surface of the
substrate 10 substantially perpendicularly to the longitudinal axis
of the optical waveguide 12. The elastic surface wave has 90% of
its strain energy concentrated within a depth on the order of a
wavelength thereof from the surface of the substrate 10. The
elastic surface wave has a wavelength .LAMBDA. defined by .LAMBDA.
= Vs/f where Vs designates the sound velocity of the elastic
surface wave, and f designates the frequency of the high frequency
voltage applied across the transducer electrodes 16. When the
frequency f is tuned to the central frequency fo of the
interdigital electrode 16, the elastic surface wave is generated
with a maximum efficiency. At that time the wavelength .LAMBDA.
thereof is equal to the finger period d of the interdigital
electrode 16.
An elastic strain wave on the surface of the substrate 10, that is,
the elastic surface wave, propagates across the optical waveguide
12 to cause a periodic variation in the refractive index of the
optical waveguide 12 due to the photoelastic effect of the surface
wave on the material of the waveguide. This periodic variation in
the refractive index functions as a diffraction grating with
respect to an optically guided wave 24 propagating within the
optical waveguide 12 through one end labelled INPUT END.
The diffraction of light resulting from such a diffraction grating
is sorted into a Roman-Nath diffraction and a Bragg diffraction.
Either of these two types of diffraction of light is characterized
by a value of a parameter Q expressed by the following
equation:
where n designates the effective refractive index for an optically
guided wave, .LAMBDA. the wavelength of an elastic surface wave,
.lambda.o the wavelength of light in a vacuum and L designates the
aperture length of a transducer such as the interdigital transducer
14 that is, the width over which an elastic surface wave such as
the wave 22 is generated and propagated. If the parameter Q is
caused to be equal to or greater than 4.pi. then Bragg diffraction
can be caused. Unlike the Raman-Nath diffraction, this Bragg
diffraction makes it possible to deflect the optically guided wave
24 to produce a diffracted optical wave 26 with a diffraction
efficiency of 100%. Therefore, causing Q to have a value Q .gtoreq.
4.pi. is preferable in order to produce Bragg diffraction and thus
produce highly efficient optical switching devices.
Upon the occurrence of the Bragg diffraction, the optical
diffraction efficiency is maximum when the angle 2.theta. between
the optical guided wave 24 and the diffracted optical wave 26
fulfils the following requirement for the Bragg diffraction:
The diffraction efficiency .eta. with which the optical guided wave
24 being propagated through the optical waveguide 12 undergoes the
Bragg diffraction is expressed by
where Pa designates a power of an elastic surface wave 24, F an
overlap integral of transverse distribution functions of the
optical guided wave 24, the diffracted optical wave 26 and the
elastic surface wave 22, and M designates an acousto-optical
figure-of-merit. If the optical waveguide 12 has a thickness W
sufficiently greater than a cut-off film thickness for the
optically guided wave, then the overlap integral F is substantially
one. The acousto-optical figure-of-merit M is a value
characteristic of the medium involved and is the most important
factor for determining the efficiency with which an optical wave is
deflected due to the acousto-optical effect. The value M is
expressed by the following equation
where p and .rho. respectively designate the photoelastic constant
and the volume density of the particular medium in which the
acousto-optical interaction is caused.
In order to provide acousto-optical deflection switches having a
high light diffraction efficiency, it is generally necessary to
cause the acousto-optical interaction within materials having a
high value of M. In other words, it is necessary to form thin-film
optical waveguides of materials having a high value of M. Any
material having a high value of M exhibits fairly high propagation
losses at wavelengths of light from semiconductor laser diodes, YAG
lasers or HeNe lasers.
On the other hand, optical waveguides can be formed of materials
capable of reducing the in propagation loss to, for example, 0.1
db/cm. Such optical waveguides are typically formed on a substrate
of fused quartz by sputtering Corning a glass having the
composition 61.0 mol % of SiO.sub.2, 10.0 mol % of Al.sub.2
O.sub.3, 19.0 mol % of B.sub.2 O.sub.3 and 10.0 mol % of BaO, one
such glass being glass No. 7059 in a thin film upon the substrate.
Any optical waveguides thus formed have a small value of M and a
light deflection efficiency as low as at most 60 percent in terms
of ordinary acoustic power.
Thus conventional optical switching devices such as above described
have not had both high light diffraction efficiency and low light
propagation loss.
Referring now to FIG. 3 wherein like reference numerals designate
components identical or similar to those shown in FIG. 1, there is
illustrated a thin-film optical switching device constructed in
accordance with the principles of the present invention. The
arrangement illustrated is different from that shown in FIG. 1 only
in that in FIG. 3 a thin film 28 of rectangular shape is disposed
on the surface of the substrate 10 so as to cover that portion of
the optical waveguide 12 located on the path of propagation of the
elastic surface wave. In FIG. 3 the thin film 28 is shown as
including a greater portion overlying the flared portion of the
optical waveguide 12 and the remaining portion thereof overlying
the substantially uniform width portion of the waveguide. The thin
film 28 is formed of any suitable material having a high value of
M, for example, chalcogenide or arsenic sulfide (AsS.sub.x) glass
where x is a value of from 3 to 7. It is to be noted that the thin
film 28 has both a width L required for effecting the
acousto-optical interaction between the same and the adjacent
portion of the optical waveguide 12 and a thickness W.sub.2
required for transferring almost all the energy of the guided light
24 from the optical waveguide 12 to the thin film 28. The
dimensions L and W.sub.2 are designated in FIG. 5.
The condition for transferring a TEo mode guided wave from that
portion of the optical waveguide 12 overlaid by the thin film 28 to
the latter is obtained from the following equation. ##EQU1##
The parameters appearing in the above equation are thus defined as
follows:
n.sub.o = refractive index of substrate 10
n.sub.1 = refractive index of optical waveguide 12
n.sub.2 = refractive index of thin film 28
n.sub.3 = refractive index of medium such as air contacted by upper
surface of thin film 28
W.sub.1 = thickness of optical waveguide 12
.beta. = propagation constant of optical guided wave 24
k.sub.o = propagation constant of light in vacuum
If the n.sub.1, n.sub.2, W.sub.1 and W.sub.2 are selected according
to e = base for Napiorian logarithms the relationship n.sub.1
<.beta.<n.sub.2 then the optically guided light 24 has the
energy thereof transferred to the thin film 28 and is concentrated
in the latter.
Also for the propagation of the elastic surface wave 22, the thin
film 28 is preferably of a material which not only has a high
acousto-optical figure-of-merit but also a lower velocity of sound
therein than the materials of both the substrate 10 and the optical
waveguide 12.
Further the optical guided wave 24 propagating through the optical
waveguide 12 has a waveguide mode as determined by the refractive
index n.sub.1 and the film thickness W.sub.1 of the optical
waveguide 12. In order to effectively transfer optical energy to
the thin film member 28, the refractive index n.sub.2 and film
thickness W.sub.2 of the thin film 28 are required to be selected
with reference to the refractive index n.sub.1 and film thickness
W.sub.1 of the optical waveguide 12 while being suitably sloped at
the upstream and downstream edges of the thin film 28 in a
direction in which the optically guided wave 24 travels within the
optical waveguide 12 as best shown in FIG. 5. It has been found
that with an optical waveguide 12 formed by sputtering Corning
glass No. 7059 (the refractive index n.sub.1 of which is 1.53 at a
wavelength of 8700 of light in a vacuum) to a film thickness
W.sub.1 of 1.7 microns on a substrate 10 made of fused quartz
having a refractive index n.sub.o of 1.452 at the same wavelength
of light, a satisfactory result is obtained with a thin film 28
formed of a diarsenic pentasulfide (As.sub.2 S.sub.5) evaporated
film having a refractive index n.sub.2 of 2.32 at the same
wavelength of light, a thickness W.sub.2 of 2000 A and a slope of
1/1000 or less.
In operation, the elastic surface wave 24 from the transducer 16
propagates from the righthand portion as viewed in FIG. 4 of the
surface portion of the substrate 10 into the thin film 28 as shown
by the meandering dotted curve in FIG. 4. The elastic surface wave
22 transferred to the thin film 28 exerts the acousto-optical
effect on the optically guided wave 24 at that time transferred to
the thin film 28 as above described. This results in the optically
guided wave 24 changing to a diffracted optical wave 26 due to the
Bragg diffraction as in the arrangement shown in FIGS. 1 and 2.
Also as shown at dotted and dashed lines in FIG. 5, almost all the
energy of the optically guided wave 24 from the righthand portion
as viewed in FIG. 5 of the optical waveguide 12 is transferred to
the thin film 28. Within the thin film 28 the transferred optical
energy is affected by the elastic surface wave 22 as above
described after which the affected optical wave is transferred back
to the lefthand portion as viewed in FIG. 5 of the optical
waveguide 12. This path of propagation of the guided wave 24 is
also shown by the dotted and dashed line in FIG. 5.
The operation of the arrangement as shown in FIGS. 3, 4 and 5 will
now be described in more detail. It is assumed that the high
frequency source 18 is in its OFF state in which the interdigital
electrode-transducer arrangement 14 - 16 generates no elastic
surface wave and therefore no surface wave is supplied to the thin
film 28. Under the assumed condition, the optically guided wave 24
introduced into the optical waveguide 12 through the righthand end
as viewed in FIG. 3 labelled INPUT END is transferred to the thin
film 28 as above described. Because of the absence of the elastic
surface wave within the thin film 28, the transferred optical wave
24 propagates through the thin film 28 without deflection after
which the optical wave 24 is returned to the optical waveguide 12
until it appears at the other end thereof labelled OUTPUT END as
the optical wave 24, undergoing no diffraction.
On the other hand, where the source 18 is in its ON state in which
the elastic surface wave 22 is being supplied to the thin film 28
and where the angle formed between the directions of propagation of
the elastic surface wave and optically guided wave 24 and 26
respectively is equal to a Bragg angle satisfying the Bragg
condition, the optically guided wave 24 after having been
introduced into the thin film 28 as in the above case is deflected
through an angle of 2.theta. (see FIG. 3) due to the Bragg
diffraction. Then a diffracted optical wave 26 appears at the
output of the optical waveguide 12 and is in a different position
from the optical guided wave 24.
In this way, the high frequency source 18 can be brought into
either its ON or its OFF state to control the supply of the elastic
surface wave 22 to the thin film 28 whereby the optical guided wave
24 introduced into the optical waveguide 12 through input end can
be switched to travel along either a path corresponding to the
direction of incidence of the optical wave and era path deflected
from the first-mentioned path at a predetermined angle within the
output portion of the optical waveguide 12 as the case may be.
As an example, an arrangement such as shown in FIGS. 3, 4 and 5
were formed of the following components: the substrate was of fused
quartz having a refractive index of 1.452 at a light wavelength of
8700 A in a vacuum and a photolithographic process was utilized to
sputter Corning glass No. 7059 into a rectangular thin film 3 on
one surface of the substrate to form an optical slab waveguide. The
optical waveguide thus formed had a thickness of from 1.0 to 1.5
.mu.m, a width from 50 to 100 .mu.m and a refractive index of 1.53
at a light wavelength of 8700 A. A thin film was formed of
chalcogenide glass, arsenic sulfide As.sub.2 Sx, where x has a
value of from 3 to 7, by vacuum evaporation. The resulting thin
film had a film thickness of from 2,000 to 3,000 A and included one
portion overlying the optical waveguide and having a length of from
2.5 to 3 mm. Also it had an acousto-optical figure-of-merit M of
about 400 X 10.sup.-18 sec.sup.3 /g, a refractive index of 2.35 at
a light wavelength of 8700 A and a slope as above described in
conjunction with FIG. 5 equal to or less than 1/1000. An
interdigital transducer such as transducer 14 shown in FIG. 3 was
formed by a thin film of zinc oxide (ZnO) to generate an elastic
surface wave at a frequency of 130 MHz with a finger period of 20
.mu.m for the associated interdigital electrodes.
When an optical wave emitted at a wavelength of 8700 A in a vacuum
from a laser diode was used with the arrangement just described,
the parameter Q was greater than 4.pi. so that the acousto-optical
interaction occurred under the Bragg condition. The resulting angle
of deflection 2.theta. between the straight guided wave and the
deflected wave amounted to 2.theta. = 1.6.degree..
Thus it is seen that the present invention provides a thin-film
optical switching device having a high light diffraction efficiency
a low optical propagation loss and coupling loss while retaining
the advantages of conventional devices. Further the present
invention can be easily coupled to and integrated with other
optical circuit elements.
While the present invention has been illustrated and described in
conjunction with a single preferred embodiment thereof it is to be
understood that numerous changes and modifications may be resorted
to without departing from the spirit and scope of the present
invention. For example, the substrate 10 may be formed of a crystal
of ferroelectric material such as lithium niobate (LiNbO.sub.3) or
lithium tantalate (LiTaO.sub.3), and the optical waveguide 12
having a high refractive index can be disposed on the surface of
the substrate 10 through the out-diffusion of lithium (Li) or the
diffusion of niobium (Nb), titanium (Ti) or copper (Cu).
Alternatively, the optical waveguide 12 may be formed of a single
crystal in the form of a thin film having a high refractive index
through the use of a liquid or vapor epitaxial growth technique.
With the substrate formed of a ferroelectric crystal, it is
required only to dispose a pair of interdigital electrodes on that
surface of the substrate having a crystal face suitable for the
propagation of sound waves. This results in the advantage that the
manufacturing process is simplified.
Further it will readily be appreciated that the thin film 28 may be
of tellurium dioxide (TeO.sub.2) glass and that the substrate may
be any of a multitude of non-piezoelectric materials having a small
acousto-optical effect, for example, fused quartz, sapphire, YAG
(yttrium aluminum garnet) etc.
Also it is to be understood that the thin film 28 is not required
to be larger in width than the optical waveguide 12 and that the
same may be just superposed on a predetermined portion of the
optical waveguide, that is, the two may be identical in width to
each other.
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